1. Field of the Invention
The present invention relates to a fiber optic device used for a pattern acquisition apparatus such as a fingerprint detecting apparatus.
2. Related Background Art
A fiber optic device disclosed in Japanese Patent Application Laid-Open No. 7-174947 has conventionally been known. FIG. 13 is a perspective view showing such a fiber optic device, whereas FIG. 14 is a partially vertical sectional view of this device 500 taken along line XIV--XIV of FIG. 13. This device 500 is typically used as means for transmitting an irregularity pattern image in an apparatus for acquiring an irregularity pattern of an object surface, such as a fingerprint detecting apparatus.
As shown in FIGS. 13 and 14, the fiber optic device 500 has a structure in which a plurality of optical fibers are bundled together so that their respective axes are in parallel to each other. The respective end faces of the optical fibers are collected so as to become flush with each other to form opposite end faces 502 and 504 of the device 500. The end faces 502 and 504 are faces to receive and to emit an optical image, respectively. The input end face 502 and the output end face 504 are made to be parallel to each other so that the optical image emitted from the output end face 504 is not distorted.
As shown in FIG. 14, each optical fiber constituting the device 500 comprises a core 512 in its center, a cladding 513 closely surrounding the core 512, and a light absorber 514 closely surrounding the cladding 513. Opposite end faces of each optical fiber are inclined with respect to an axis 518 of its core 512 at an angle .alpha..sub.0, and thereby the input and output end faces 502 and 504 are also inclined with respect to each core axis 518 at an angle .alpha..sub.0. In other words, respective normals 506 and 508 of the input and output end faces 502 and 504 are not in parallel to each axis 518 but each form an angle of (90.degree.-.alpha..sub.0) with respect to each axis 518. This angle of inclination .alpha..sub.0 is set to a value at which, even when light rays enter each core 512 from the air, these rays are not totally reflected at an interface between each core 512 and cladding 513. Namely, the angle of inclination .alpha..sub.0 of the input and output end faces 502 and 504 is included in such an angle range that the incident angle of each light ray entering each core 512 from the air with respect to an interface between each core 512 and cladding 513 is not greater than the critical angle of reflection at this interface.
As is well known, such a range of angle of inclination .alpha..sub.0 can be represented as .alpha..sub.0 .ltoreq..alpha..sub.0m, where .alpha..sub.0m is a specific maximum angle of inclination. This .alpha..sub.0m is an angle satisfying the following three equations:
n.sub.0core .multidot.sin .phi..sub.c =n.sub.0clad .multidot.sin 90.degree. (Snell's law at the interface between the core and the cladding), PA1 n.sub.0core .multidot.sin .beta..sub.0m =n.sub.a .multidot.sin 90.degree. (Snell's law at the interface between the air and the core), and PA1 .alpha..sub.0m +(90.degree.+.beta..sub.0m)+(90.degree.-.phi..sub.c)=180.degree. (sum of interior angles of a triangle),
where n.sub.0core is a refractive index of each core 512; n.sub.0clad is a refractive index of each cladding 513; n.sub.a is a refractive index of the air; .phi..sub.c is a critical angle of reflection at the interface between each core 512 and cladding 513; and .beta..sub.0m is an angle of refraction of an incident ray 520 on the input end face 502 at an incident angle of 90.degree.. More specifically, .beta..sub.0m is an angle formed between the normal 506 of the input end face 502 and a refracted ray 521 of the incident ray 520.
By using .alpha..sub.0m determined from the above three equations, the range of angle of inclination .alpha..sub.0 can be represented as: EQU .alpha..sub.0 .ltoreq..alpha..sub.0m =sin.sup.-1 (n.sub.0clad /n.sub.0core)-sin.sup.-1 (n.sub.a /n.sub.0core). (1)